Smart phones -- cellphones with a big screen, a keyboard and the ability to run add-on programs -- are not necessarily phones to the people who use them. Their versatility allows them to impersonate the other handheld devices you might otherwise carry around; in reality, any given smart phone may be a datebook, a Web browser or an MP3 player that happens to place the occasional call.

Then again, it's hard to find a device equally competent at all of these possible uses, as a trial of four recent models showed.

These four gadgets -- Nokia's E62, Palm's Treo 680, Research in Motion's BlackBerry Pearl and T-Mobile's Dash -- bear a superficial resemblance. All can fit in a pocket, though the wide E62 and the relatively thick 680 do so less gracefully than the other two.

They also cost less than many other smart phones -- at least until the bills start coming in. Cingular sells the E62 for $200 and the 680 for $300, with $100 rebates on both. But voice and data use start at a combined $80 a month.

T-Mobile, meanwhile, sells the Dash and the Pearl for $250 each. The Dash carries a $100 rebate, with voice and data service starting at $60; the Pearl has a $50 rebate and voice/data service from $50. On Friday, Cingular began selling the Pearl as well, at the same price as T-Mobile.

As a phone, the Treo works best: You can turn its phone on or off, silence the ringer, activate its speakerphone or lock its keys with the press of a single button or switch.

The other devices all gum up phone functions by forcing you to navigate though on-screen menus, though the BlackBerry was less compromised than the others. Its key-guard button is right on the top, and its silent-mode command is only a couple of button presses away.

All four phones included Bluetooth wireless connectivity, but the Dash was the only one that could use the hands-free capability of a Bluetooth-equipped Toyota.

As an organizer, the Treo and the Dash almost tie each other. The Treo can synchronize its address book, calendar, to-do list and memo pad with more software, either the simple but antiquated Palm Desktop or Microsoft's Outlook. But the Dash's Windows- and Outlook-only ActiveSync software is faster and simpler than Palm's HotSync.

Microsoft has boiled much of the original complexity out of its Windows Mobile system, making most tasks on the Dash about as quick as they are on the Palm. Unfortunately, the Dash's smart-phone edition of Windows Mobile leaves out the essential ingredient of a notepad application. And hardly any third-party programs have been released for that version of Windows Mobile, leaving the Dash ultimately less capable than the Treo.

The BlackBerry's ingeniously miniaturized keyboard, in which two letters appear on each key and the software figures out what your key presses would most likely spell, worked poorly for any extended data entry. Research in Motion seems militantly opposed to simplifying its device's interface: No task is too simple to have irrelevant menu choices or are-you-sure? prompts thrown in its way. Synchronization was sluggish, and few extra programs are available for the Pearl.

Nokia's keyboard was wonderfully roomy, but its Symbian operating system ran sluggishly, with brief but aggravating waits when switching between programs. And Nokia's desktop software was downright grotesque.

As a Web browser, the Dash and the Nokia each stood apart. The Dash could connect both via T-Mobile's relatively slow EDGE data service and much faster WiFi wireless networks. And its miniature version of Microsoft's Internet Explorer did a decent job of displaying Web sites as they'd look on a "real" computer.

The Nokia's browser did better yet, incorporating a crafty solution to the problem of having to scroll around pages that don't fit on its screen: It pops up a transparent overlay that shows a miniature version of the page, with the part that you're viewing set off in a red outline.

The Treo's Web browser is better suited to pages written for its smaller size; it can display full-size sites, but you wouldn't want to spend much time doing so. The BlackBerry's browser, meanwhile, can do almost everything the Treo's can, but looks a lot uglier in the process.

As an e-mail reader, consider the Treo, followed by Dash or Nokia-- not the BlackBerry. The BlackBerry may have the reputation of being the chosen mail device of the ruling class, but it stumbled with home e-mail accounts. Setting up the Pearl to check one required calling T-Mobile for help -- the phone's software wouldn't go to the special Web page where I was supposed to enter the account info. Turning off incoming mail delivery takes too many steps, and messages that weren't plain text often showed up garbled.

The Treo, meanwhile, comes preset with configuration info for many popular mail services; all you need to do is key in your user name and password. You can check your mail as often or as rarely as you like, and messages show up with their formatting intact.

The Dash and the Nokia took more work to configure than the Treo but still functioned better than the BlackBerry.

As a multimedia player, the Dash just beats the Treo. It has a higher-resolution camera and more capable photo-viewing software, while its Windows Media Player software takes less work to synchronize with a computer's music collection than the Treo's Pocket Tunes.

The Treo, on the other hand, doesn't make you remove the battery cover to get at its storage-card slot: it's right on the side, behind a plastic flap. The Treo was also the only device in this group to use standard-size SD Cards.

The Nokia lacks a camera and conceals its memory-card slot under the battery cover. The Pearl does include a pretty good camera and music-playback software, but it makes it even harder to add a memory card full of MP3 files -- the card slot is under the battery itself.

Of all these phones, the Dash feels closest to any sort of universal competence -- but maybe that's just because Microsoft has put so much more effort into improving its software over the past few years.